Process of inhibiting the hydration of anhydrous calcium sulphate



ferent phenomenon. The crystal form of the.

' chemical substances.'

,-;um1-E Q'STIATES "PA T;

- 2,259,638 PROCESS or nnnnrrmo THE momma OFANHYDROUS CALCIUM sULPna'm- 1 Winfield Walter Heckert, Ardentown, DcL, as- 7 signer to E.I. duPont de Nemours & Company,

.. Wilmington, Del., a corporation of Delaware NoDrawing.

pplication August 1c, 1938, .Serial'No.225,716

9 Claims. "(cr es-soc) The present invention relates to 'a treatment of the insoluble anhydrite modification of calcium sulphate with awater soluble protein-which l effects a stabilization of the anhydrous calcium sulphate against hydration. v

This application is a continuation-impart of my co-pending application Serial No. 19,168, filed May 1,1935. 'The insoluble anhydrite modification of calcium sulphate is characterized by a specific X-ray 4 diffraction pattern. The natural occurring anhydrous calcium sulphate is of the insoluble type. It can also be obtained-by the calcination of plaster of, Paris or gypsum, etc., at temperatures above for instance 500 C. Insoluble anhydrite m1 when the gypsum is dehydrated in am;

,sulphuric'acid. It is in this case necessaryto separate the product formed from the reaction mediumwhich operation comprises washing the product free from acid, filter pressing and drying it. During the latter stages-of ft'he'washing" step, when the acid concentration is very low and during the pressing-operation, bothof which crystals is frequently observed,

can also be produced by the dehydration of.

gypsum in-a strong sulphuricacid, or bygradual- 1y adding slaked lime to about 609 Baum sulphuric acid and continuing the reaction until approximately 85%'of the acid-becomes neutralized. .The present invention relates to the stabiliza- I tion-of the insoluble anhydrite modification of calcium sulphate, and more especially to the precipitated, finely-divided variety 'useful as a "pigment or pigment extender, i. e., anhydrite are apt to be time consuming "on a plant scale, there' is an opportunity for hydration ,ofv the original anhydrite and formation of This formation of gypsum is readily prevented if the anhydrite slurry'and the washwaterjcom tain a small amount of gelatin, glue or other water-soluble colloidal protein. vAmounts found very convenient are, for instance, 0.1% gelatin, based upon thewelght 1 0i the anhydrite, or. 0.025%, based upon the solution, .in general the glue is effective with amounts above 0.01%, a practical upper-limit being 1%; f The pH or .the

1 gluesolution in which the calcium sulphate iswhich lsnot only of satisfactory pigmentary' color, but of requisite frequency particle size average, that is, 'at least 75% of its particles range preferably within about .5 to 2 microns and none exceed about.5 microns. The invention consists in submitting the finely-divided anhydrite to an aqueous solution of a' protein.

Gelatin, glue, egg albumin and other albumi-' noids were found by me to exert a stabilizing influence upon insoluble anhydrite in contact with an. aqueous medium, in that there is a decided slowing down of the hydration of the calcium sulphate.

more.

I- am aware that glue and gelatin have been used to prevent the formation of needle-like crystals in precipitatinggypsum from an aqueous medium, lt-had namely been found (see U. S. 879,603 of Feb. 18, 1908, M. 'Wallerstein) that when one adds gelatin to an aqueous'solution'of calcium chloride and reacts upon this withsodium sulphate, the precipitated gypsum will be in the shape of relatively large flakes or tabletlike crystals instead of the needles formed in the absence of the gelatin.

invention isdirected to an entirely dif- 'siders that anhydrite and gypsum are different The, present application is particularly usesuspended has also a certain effect upon the-in.- hibiting efie'ct of the protein. In general, itlis preferred to maintain the reaction of thej sysr tem at a pH of not less than 4'up to 10 or even The protein material maybe adsorbedfiuponthe particles of anhydrite and form a film there-- on. In any case, some protein remains'associ-- ated with the anhydrite after drying.

In cases where the anhydrite is produced by dry methods, it can be protected against 'sub- I sequent hydration upon being brought intocontact with water by mixing withthe-dry. anhydrite a small percentage ct water-soluble protein, such as glue, albumin, etc.

The specific efl'eet of glue, gelatin, and other protein colloids in inhibiting the hydration of anhydrite was shown in the following experi-. merits: p i

A 20% aqueous slurry of fresh y precipitated anhydrite was adjusted to a pH of 8 with caustic v soda and divided into 3 portions. The anhydrite contained already 4.6% of the C8804 as'gypsum.

One part was used ascontrol. To the "other two parts 0.05% and 0.1% animal glue (based on the CaSOi) was added respectively. After 5 days standing the following results were obtained:

No glue rcent gypsum .-10.'l. 0.05%. gluedo 4.8 0.1% glue 4 5 do 4.7

CaSO4) was added to the other.

as percentage conversion to gypsum:

the eifect of the pH of the slurry was observed asfollows:

Table 1 Gypsum Gypsum at start eiter7days Percent Percent pH5 3. 66 10. 2 nmz 3.65 8. 7 pH5.-. 3.66 4.6 pH8-. 0.1 glue 3.66 4.3

In a plant operation 5600 lbs. of anhydrite in 20% water slurry was accumulated in a large tank. .The slurry was adjusted to pH 8 with sodium hydroxide and a sample removed.

5.6 lbs. of animal glue dissolved in 15 gallons of hot water was then run into 'the tank and allowed to stir in. Hydration data were ohtained as follows: Table 2 G sum ,G sum az tart Mt days Percent Percent Control sampl e 3. 3 6. 2 Anhydrite in tank. "1.; 8. 3 3. 4

has been prepared under conditions which avoid the presence of appreciable quantities of the hydrated forms oi calcium sulphate in the initial slurry, and to demonstra the negligible extent of hydration occurrin upon treatment in cium sulphate hemihydrate, whereas I have found a accordance with my invention, the following example is given:

, A 20% aqueous slurry of freshly precipitated anhydrite was adjusted to a pH of 8 with a small amount of milk of lime and divided into. 2 parts. One of the portions was used as a control while 0.10% of animal glue (0.10 grams per 100 grams The two sampleswere observed for hydration over a period of 10 days with the following results expressed Table 3 Time (days) Percent vPercenl Percent Percent Control 2. 4 0 14. 6 26. 7 63. 3 Glue treated -s. 0. 05 V 0- 1 4 0. 6

As will be evident, my invention aflords the production of pigment-useful insoluble anhydrite 1 desirably and efiectively stabilized against hydration. The treated calcium sulphate particles will be found to be relatively free from any tendency to grow or increase in size when in contact with aqueous media, and especially during. pigment washing or other processing, when prolonged of gypsum with consequent increase, in particle size of the calcium sulphate deleteriously aifects the adaptability of the anhydrite for pigment used, since gypsum presence results (1) in poor I texture and (2) inloss in tinting strength and hiding power of the pigment, the retention of both of which is obviously desirable, and in fact necessary, to render the pigment commercially adaptable.

I am also aware that use of proteids for delaying or retarding the hydration of soluble anhydrite and plaster of Paris or hemihydrate calcium sulphate, has already been proposed. However, any such degree of stabilization as is required for commercially-acceptable, pigment grade, insoluble anhydrite does not result from such treatment. Thus, even when resort is had to concentrations of proteids ranging to as high as ten times the amount which I normally em-.,

ploy for efiecting relatively complete stabilization of pigment grade insoluble anhydrite, hydration of soluble anhydrite or hemihydrate calcium sulphate goes substantially to completion, to yield stubby, hexagonal crystals of gypsum. This difference in behavior is apparently related to the difference in crystal structure between these forms of calcium sulphate.- X-ray examination indicates that the structure of soluble anhydrite is quite similar to that of calcium sulphate hemihydrate, whereas that of insoluble anhydrite is markedly different. 'Thelstability'to hydration -of.,proteid-treated, pigment grade,.insoluble anhydrite is still more surprising, in view of its very fine particle size, niuchiiner than, for example, flooring plaster which would 'be entirely unsuitable for' pigment, use because of its large particle size. Small particle size should favor hydration and thus make the problem 01' stabilizing against hydration much more dimcult in the case of pigment grade, insoluble anhydrite than in case of plasters made from soluble anhydrite or calthe reverse to be true:

I claim as my invention: 1. m a process of producing pigment-useful,

'insoluble anhydrite at least 75 per cent of the particles of which have an average particle size ranging from substantially .5-2 microns, which involves the production through precipitation of insoluble anhydrite in contact with sulphuric acid and the washing of said insoluble anhydrite with L an aqueous medium, the step of inhibiting the hydration of said insoluble anhydrite which comprises contacting it with about .01% to 1%, based on the weight of the anhydrite, of a watersoluble protein prior to separation of said' anhydrite from said aqueous medium. r

2. In a process of producing pigment-useful, insoluble anhydrite at least 75 per cent of the particles of which have an average particle. size ranging from substantially .5-2 microns, .which involves the production through precipitation of insoluble anhydrite in contact with sulphuric acid and the washing of said insoluble anhydrite with an aqueous medium, the step of inhibiting the hydration of said insoluble anhydrite which comprises contacting it with about .01% to 1% of completed. j x

' 3. In a process of completely stabilizing pigment-useful, precipitated insoluble anhydrite at least 75 per cent of the particles of which have, an averageparticle size rangingfrom substantially .5-2 microns, against hydration, the step of contacting said anhydrite with an aqueous ration from aqueous sulphuric acid dehydratwhich comprises during conversion by dehydra v ticle size ranging from substantially ,6-2 mlfrom said anhydrite, withan aqueous solution ing the -resulting stabilised product. 7 r insoluble anhydrite against hydrationyat least of the particles or which have an average parl-- microns and none exceeding about 5 microns, I

- hydration-oi ahydratrigrm of calcium sulphate contacting said precipitated anhydrite,-while in through sulphuricaci anhydrite while in the presence oi said sulfuric so prior to complete removal oi said acid media glue-treated anhydrite.

. microns, which comprisesprecipitating insoluble average size ranging from substantialiy;..5-2 mimedia; contacting -said ..preclpitated anhydrite, comprises precipitating" insoluble anhydrite by and prior to complete relnoval of said acid from I sulphuric acid, co tinuing the reaction until 'apsaid precipitated anhydrite, with from about proximately 85% or the acid becomes neutralized All-1% oirglue. and recovering the resulting 85 contacting said precipitate, while in the presence stabilized glue-treated anhydrlte.

v microns .and none exceeding about 5 microns.

in: medium. H tion of a hydratediormoi calcium sulphate 4. .A process iior stabilizing against hydration through sulphuric acid treatment,lcontacting said pigment-useful; insoluble anhydrite. at least 75% anhydrite, while in the presenceoi said sulphuric of the particles of which have an average par- 5' acid and prior to complete removal of said acidcrons, which comprises contacting said anhydrite containing glue in an amount ranging from after its formation in the presence oi sulphuric about .01-l%, based on the anhydrite, and then acid media and prior to complete removal of recovering the resulting, stabilized, glue-treated, said media from the anhydrite, with irom about 10 anhydrite. V 1 a .0l-l% oia water-soluble protein, and recovera 8. A process jor stabilizing pigment-useful;

5. A process for stabilizing against hydration 75% 01' the particles of said anhydrlte having I pigment-useful; insoluble anhydrite. at least an average size ranging from substantially .5-2

ticle size ra'nging'iroiii- 'substantlally.5.-2 miwhich comprises precipitating insoluble anhycrons, which comprises durlng'conversion by dedrite by" reacting lime with sulphuric acid media,

'atment, contacting said the presence oi said sulphu'rlcacid media and acid and prior to comp et'e removaloi said acid from said ,anhydrite, with an aqueous solution from said anhydrite, with mm about .01-1% containing glue in an amount ranging from about i or glue, and recoverin 'the resulting stabilized .01-1%, based on the anhydrite, and then re I covering the resulting stabilized glue-treated [8. A- process for stabilizing pigment-useful, gsanhydrite. Y insoluble. anhydrite against hydration, atleast 9. A process for stabilizing pigment-useiful, 75% of the parti les oisalid anhydrite having an insoluble anhydrite against hydration, at least average size ranging irom substantially .5-2 of the particles of said anhydrite having an anhydrite by r'eactingdl'me with sulphuric acid so cons and none exceeding about. 5'microns, which whilein the presence oislaid sulphuric acid media gradually adding slaked lime to about as.

' or said acidic and in a washing medium; withan I. A process for stabilizing pigment-useiui, aqueous solution containing glue in an amount insoluble anhydrite-agai'nst hydration, at least ranging irom ill-1%, based on the anhydrite.

% of the particles oi said anhydrlte having and then recoverihg. the resulting. stabilized an-average size ranging from substantially. .li-'2 glue-treated anhydrite.

- HECKERT. 

